GEOGRAPHICAL DISTRIBUTION AND DESCENT. 



179 



the porcupines make their first appearance, 

 and the piping* hares have been found associ- 

 ated with an extinct genus, Titanomys, which, 

 during the Miocene period, inhabited Central 

 Germany and France. 



To these families, which are continued 

 right on without interruption, the Pliocene 

 strata add the families of the chinchillas and 

 the Subungulata in South America. The 

 other genera and families are confined in that 

 period to the regions now inhabited by them, 

 and though a great number of still surviving 

 or extinct genera have been found in brec- 

 ciated Quaternary strata or in caves, it must 

 nevertheless be admitted that the geographi- 

 cal limits were then precisely the same as they 

 are now. 



From this series of facts it results that the 

 rodents have mainly been developed in the 

 Miocene period, but that their roots reach 

 as far back as the Eocene. Some American, 

 and indeed, perhaps, even some South 

 American types appeared in Europe at that 

 time. I do not know whether the characters 

 are clearly enough marked to establish this 

 identity satisfactorily. 



Is it possible to fix with any degree of con- 

 fidence on any old type which might be re- 

 garded as related to the rodents? The answer 

 to this question must for the present be given 

 in the negative. We can only admit the 

 possibility that the characteristic dentition of 

 the rodents has developed from that of very- 

 ancient insectivores or marsupials, possibly 

 connected with Plagiaulax, the remains of 



which have been found in the Purbeck Beds 

 (Wealden Series — Cretaceous), and recently 

 in the neighbourhood of Reims in France. 

 The occurrence in the Eocene of rodents with 

 tubercled teeth having distinct roots, and an 

 insectivorous type of structure, is in favour of 

 the first supposition, as well as the fact that 

 the aye-aye has in the milk dentition canines 

 which are afterwards shed in order to give 

 place to a dentition of a rodent type. The 

 same thing might indeed have occurred in 

 former geological epochs, and since it often 

 happens in the case of certain insectivores 

 that the canines are replaced by larger incisors, 

 the inference might be drawn that the denti- 

 tion of the primitive rodents had been modi- 

 fied through the loss of the canines. On the 

 other hand, I confess that the jaws of Plagi- 

 aulax are, in my opinion, much more similar 

 to those of the rodents than to those of any 

 other type, and that in them I cannot discern 

 the marks of a carnivorous dentition such as 

 Owen professes to have found. The large 

 incisors of this genus of the Purbeck Beds 

 have certainly no resemblance in form to 

 those of a rodent, inasmuch as they are cylin- 

 drical and pointed; but these teeth are fol- 

 lowed by a wide interval or diastema, and the 

 premolars themselves have a great resem- 

 blance to the cheek-teeth of certain rodents, 

 not only in virtue of their position, but also 

 on account of the fact of their being composed 

 of series of transversely placed enamel plates, 

 which get worn away in much the same way 

 as the cheek-teeth of rodents. 



